Selective mode filters



Filed Nov. 1, 1957 MQQDOW IN [/5 N TOP 5. E. MILLER wrgm.

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ATTO RNEV United il atent Office 2,940,057 Patented .June 7, 1960SELECTIVE M03313 PETERS Stewart E. Miller, Middletown, Ni, assignor toBell Telephone Laboratories Incorporated, New York, N.Y., a corporationof New York Filed Nov. 1, 1957, Ser. No. 693,973 9 Claims. c1. ss3 1sThis invention relates to selective mode filters for use withcylindrical waveguides transmitting circular electric wave energy.

In the transmission of electromagnetic wave energy through a hollowconductive pipe or other waveguide, it 'is well known that the energycan propagate in one or more transmission modes, or characteristic fieldconfigura- Ltions, depending upon the cross-sectional size and shape oftheparticular guide and the operating frequency, and that the larger thecross-section of the guide is made the greater is the number of modes inwhich the energy can propagate at a given operating frequency. Verygenerally .it is desired to confine propagation of the energy to oneparticular mode, chosen because its propagation characlteristic-s arefavorable for the particular application involved. If the desired modehappens to be the so-called dominant mode,it is feasible to restrict thecross-sectional "dimensions of the guide so that no modes other than thedominant modecan be sustained therein. This expedient is not availablehowever, if the desired mode is not the dominant-mode or if a guide oflarge cross-section is prescribed in order, for example, that advantagemay be taken of its relatively low attenuation. This is particularlytrue-of systems employing the TE circular electric mode. As is wellknown, .the propagation of microwave energy in theform of the TB mode incircular waveguides is ideally suited for the long distance transmissionof high frequency wide band signals since the attenuationcharacftcristiczof this transmission mode, unlike that of other modes,decreases with increasing frequency. However, since the TE mode is notthe dominant mode supported in-a circular waveguide, energy may be lostto other modes that are alsocapable oftransmission therein.

.'In an ideal waveguide which is perfectly straight, uniform andconducting, the propagation of T E waves there- .through is undisturbed,but slight imperfections in the guide and especiallycurvature of thewaveguide axis may excite waves of other modes and produce seriouslosses. These losses are attributed mainly to the fact that the bendingof the guide produces a coupling between the desired TE mode and otherundesired modes, mainly the TM mode.

Recognizing that the coupling between these modes may-be likened to thecoupling between traveling waves ontcoupled transmission lines in thatan exchange of energy willtake place between the waves when they traveltogether in media in which they have the same propagation constants, theprior art has provided a large number of devices for modifying thepropagation constants of the transmission path with respect to one ofthe modes.

In.the copending application of J. R. Pierce, Serial No. 416,315, filed.Mar. 15, 1954, now U.S. Patent 2,848,695, issued Aug. 19, 1958, it isdisclosed that a helical conductor of diameter greater than 1.2 freespace wave- :lengths will propagate a properly excited circular electricIE mode with 'a-ditferent phase constant than the TM mode.- ,Thisprovides a substantial decoupling between nate any spurious non-circularmodes.

these modes. In my copending application, Serial No. 416,316, filed Mar.15, 1954, now US. Patent 2,848,696, issued Aug. 19, 1958, it was shownthat if the respective TE and TM modes propagated along the resultinghelical transmission path are exposed in a special way to electricallydissipative or lossy material so that they have substantially differentattenuation constants, the coupling between the two modes may be madearbitrarily small. The attenuation constant of the TM mode may besubstantially increased, for example, without increasing that of the TEmode by surrounding the helical guide with a cylinder-like casing ofdissipative material having an inside surface contiguous with theoutermost part of the helical conductor. Thus, the longitudinalcurrentsof the TM mode must pass through the lossy materiaLgiving tothat wave a large attenuation constant while the component of thecircumferential currents of the TE mode which follows the helixconductor is unaffected.

Where the transmission path is constructed substantially as outlinedabove, the conversion of TE wave energy to other spurious modes may bemade negligibly small. However, it is often not feasible to providehelical waveguiding means for the circular electric mode. It may,instead -be necessary or desirable to use solid-cylindrical pipe as thetransmission medium. Where this is so, it will be essential to interposeat regular intervals, mode filtering means for the purpose of removingthose spurious modes that will be inevitably generated in any practicalsystem thus minimizing the likelihood of conversion-reconversiondistortion efiects. It would appear reasonable to construct such a modefilter in the form of a segment of helical'waveguide as described aboveto elimi- However, it has been found that merely to insert segments ofhelical waveguide between portions of solid wall guide had the effect ofgenerating additional spurious modes by virtue of the conversion of theTM mode to 'I'M TE and TE modes. This conversion takes place at thejunction of the solid and the helical waveguide and is a result of thechange in transmission characteristics experienced by the TM mode atthis point. While the additional modes thus generated will also "bedissipated in the mode filter as well as the TM mode, it has been foundthat to achieve the desired spurious mode level at the outputof thefilter, a filter of longer length is needed than would be needed if theTM mode was the only spurious mode present.

It is, therefore, an object of thisinvention to couple, with minimummode conversion, non-circular electric mode Wave energy in a multimodesystem from one transmission medium to another having differenttransmission constants.

It is an additional object of this invention to produce such couplingover a broad frequency band.

It is a further object of this invention to produce such couplingwithout interfering with the desired TE circu lar electric mode.

in accordance with a specific embodiment of the selec 'tive mode filterof the present invention, to be described is tight wound at one end, butin which the pitch gradualby increases until the spacing between turnsis approximately one-half wavelength of the lowest frequency to betransmitted. At this point the second helix is discontinued. Thedissipative jacket comprises a cylinder-like casing having aninsidesurface contiguous with the outermost part of the helical conductors.Thus, the longitudinal currents of the TM mode, which must pass 7affected.

' lossy material, are gradually exposed to the lossy material, therebycausing this wave to experience a 7 smoothly increasing attenuationconstant, while the comconversion is minimized. The filter length iscalculated w to reduce the spurious mode level the systernto a specifiedlevel and is then discontinued, returning to the solid wall guide. Sincethe transmission characteristics a of both the filter and the solid wallguide are the same for the circular electriemodes, there is no need forany 'matchingi'mea n s ateither end of the filter for these modes. Ifthe system is limited to transmissions in only one direction, it issufficient to have only'one taperedhelical member at the input end ofthe filter. If however, wave energy is'to be transmitted in bothdiIections sirniIar tapered helical'members must be provided atboth endsof the filter. I 7

These and other objects, the nature of the present invention,and'itsvarious features and advantages, will appear 'ni'ore'fully upon moredetailed consideration of the specific illustrative embodiment shown inthe accompanying drawings and analyzed in the following detaileddescription of these drawings.

In the drawings: 1 a Fig. 1 is a perspective view of a wave transmissionsystem employing a helical filter in accordance With the principles ofthe invention; and

' Fig 2 is apartrally cut away view of the helical filter shown in'Fig. 1. g I

Referring'more particularly to 'Fig. l, there is shown anelectromagnetic wave transmission system comprising a source 11 of TEcircular electric microwave energy connected to load 12 by means ofsolid wall waveguide transmission path 13 of substantially circularcross-section. Load 12 is-adapted to utilize TE circular electric modewave energy, while transmission path 13 represents thetype oftransmission line having bends,"joinits,slight physical imperfections,and deliberately inserted components, any of which tend tointroduce modeconversion effects therealong'. As is well known, these efiectsconvertportions of the TEg wave into'n'on-circular spurious waves, particularlythe TM mode. i To prevent undue reflections and otherdeleterious'etlects, these spurious modes must .be eliminated in somemanner prior to'enter- ,ing load '12;

In accordance with the present invention; the spurious modesintroducedby path 13 are eliminated from the output thereof by'a filter14icomprising the helically wound conductors 15 and 16 surrounded andsupported by a lossy jacket 17.' If the distance between source 11 andload 12 is great, more thanone filter is used. In practice a filter isplaced at regular intervals of from to 1 00 feet. 'For the purposes ofillustration only one filter is shown between the source and 1am; 1

Fig. 2 shows in detail the construction of the filter embodying theprinciples of the'invention. The inner member 15 comprises a conductorwound as a' helix having a uniform pitch and an internal diameter dequal to theinside diameter of cylindrical guide 13. Conductor 15 may besolid or stranded and may comprise a base metal such as iron or steelplated by a highly conductive material such as copper or silver.Adjacent turns 19 and 20 of the helix are electrically insulated fromeach other by either leaving a small air gap 21between adjacent turnswhen bare wire is used, or by using enameled or plastic insulated wire.Conductor 15 may, for example, be a number 37 size, enameled orplasticinsulated solid copper wire'(0.005 inch overall diameter). Byclose winding the helix, a uniform and appropriate spacing be tweensuccessive turns is provided by the insulation. The pitch angle of thehelix, should be as small as possible consistent with theabove-mentioned insulating requirement. This distance in any event mustbe less than onequarter wavelength and is preferably such that'the gap21 between adjacent turns is less than the diameter of conductor 15.

'space'between successive turns increasing with each successive turnuntil the space therebetween is greaterthan one-half wavelength of thelowest frequency to be transmitted. The longitudinal extent of helix 16is represented in Fig. 2 as e, and is between one-half to one heatwavelength, a beat wavelength being defined as MM la-M where A is thewavelength of the exciting mode andk is the wavelength'of an inducedspurious'mode; For example, k could be the Wavelength in the filter ofthe TM mode and A could be the wavelength in'the filter of the 'TE mode.Calculations must be made ofjall possible combinations of exciting andinduced modes to assurethatthe proper helix length is obtained. However,experience will generally indicate to ones'killed in the art which ofthe spurious modes are of importance and which are not, thus the numberof calculations are-generally very small. In a typical 2-inch diametertransmission system, the length of helix 16 will be about one foot.

Conductor 16 maybe in all physical respects similar to conductor 15except that it has a larger diameter. A typical wire. diameter for 16would beapproximately four times the diameter of 15. a a

If the system is limited to transmissions in only one direction, it isonly necessary to have a tapered helical member at the input end ofthefilter. If, however, wave energy is to be transmitted in bothdirections, a second tapered helix 18 is placed on the other end of thefilter. Unless considerations dictate otherwise, .helix 18 identical tohelix 16. V V The space between adjacent turns of helix 15, 16; and 18,are exposed toelectrically dissipative or lossymaterial. This may bedone by enclosing the helices Tina cylinder-like casing 17 of materialhaving'a high electrical loss. Casing 17 may be made of any suitableplastic ,or dielectric material, such'as polyethylene, in

which small particles 23 of resistive material, such as iron dust orcarbon black, are suspended. It is not desirable that'the materialofcasing 17 extend into the space between adjacent helix turns andtherefore casing '17 preferably has a smooth internal surface ofdiameter substantially equal to the outside diameter ofhelices 16 and 18where they extend over helix 15 and the outside diameter of helix 15over theregion between 16 and 18.

that the cross-section of helix 15 be maintained as nearly circular aspossible. This condition may be maintained by the resilience of casings17 and 22. It may, however, be maintained by employing for the conductorof helix 15 a spiral of spring steel plated by a highly conductivematerial. Thus, the efiectof the spring itself willrnaintain thedesiredcircularity and shield 22 may then be 'wound of overlapping, thinstrips or may be made of a woven braid. Other means of constructing thehelical filter are disclosed in a copending applicationbyKohman et.al.,Serial No. 679,835, filed August 23, 1957.

The inside diameter d of helix 15 is equal to the diameter of the solidwall circular guide 13 transmitting waves of the same frequency. Thisdiameter d must be greater than the critical or cut-ofi diameter for theTE mode in the circular guide. This cut-ofi diameter is equal to 1.22%,where A is the wavelength-in free space of the longest wave in thetransmission band. In practice, d might be in the range 1.25 to timesthe cut-off diameter, depending onthe transmission loss desired.

In operation, the circular electric TE wave being transmitted alongguide 13 will be excited within helix 15 asit enters the filter section.A major component of the circular current associated with this mode isconducted along the helical path by each turn. Since the pitch ofthe'helix is small, this component constitutes substantially the entirecurrent of the wave. A very small component of the wave is presentedwith a small reactance caused by the discontinuity between adjacentturns. This reactance will have the effect of changing the phasevelocity of the total wave very-slightly. Very little of the total TEcurrent will pass through the resistive material of casing 17 andtherefore the attenuation constant of the TE mode is changed very littleand the filter will not afiect the latter to any substantial degree.

With respect to the TM mode however, the picture is quite different. TheTM mode has a predominantly longitudinal current fiow along the walls ofthe waveguiding path and will, as a consequence, be seriously affectedby any discontinuities between adjacent turns of helix 15. By forcingthe longitudinally flowing currents to pass through the dissipativematerial of casing 17 exposed between successive turns of the helices,the eifective attenuation constant of the wave-guiding path to the TMmode energy will be greatly increased. While this is the purpose of thefilter section, unless this increase is brought about gradually, the 'IMmode will degenerate into other spurious modes having smallerlongitudinal current components. Since the attenuation per unit lengthis directly related to the strength of the longitudinal component ofcurrent, these spurious modes would not be as effectively attenuated ina helical filter and consequently a longer filter section would berequired.

The extent of exposure of the longitudinal currents to the dissipativematerial, and hence the attenuation, is a function of the ratio of thespacing between turns to the diameter of the wire, and for a fixedspacing such ratio is inversely related to the wire diameter. Largerwires present greater turn-to-turn capacitance and efiectively shieldthe inner region from the outer region. It is for this reason that helix16, used to couple the non-circular modes to the mode filter is made oflarger siZe wire. By close winding the larger diameter wire over the endof the filter, the exposure of the casing 17 to the longitudinalcurrents is initially very small. As the pitch of helix 16 opens up,more and more of the lossy jacket is exposed until the second helix isdiscontinued, at which time, the dissipative efiect of the lossy jacketis a maximum. The eifect of helix 16 might be likened to an aperturewhich is slowly opened, allowing more and more of the dissipating effectof the resistive casing to become effective. By extending the apertureopening over a length equal to from one-half to one beat wavelength,moding is reduced to a tolerable level within a reasonably shortdistance.

An alternative embodiment of the present invention uses a resistancefilm in between the lossy jacket and the helical waveguide instead of atapered helix. The resistance film acts as a shunt impedance to thesurface impedance of the lossy jacket. By varying the resistivity of thefilm, any prescribed distribution of surface resistance may be realized.Thus, the transition from the high isotropic conductivity of the copperwall to the low and anisotropic conductivity of the helix and lossyjacket structure may be made either gradually or in steps to reducespurious mode conversion in the filter.

In allcases it is understood that the above-described arrangements areillustrative of a small number of tire many possible --specificembodiments which can represent applications of the principles of theinvention. Numerous and varied *other arrangements can readily-bedevised in accordance with these principles by thoseskilled in the artwithoutdeparting from the spirit and scope of the invention.

Whatis claimed is:

' l. 'A selective mode filter for the preferential transmission ofcircular electric wave energy comprising -'a first means for propagatingcircular electric and noncircular electric modes of wave energy, saidfirst means having afirst and a "second attenuation constant for saidmodes respectively, second means coupled tosaid first means forpropagating said circular electric and said nonacircularelectricmodewave energy, said secondm'eans havi'n'g a third and -a fourthattenuation constant for said modes I respectively, :and third meanscoupled to said first; and saidisecond :means 10 vary graduallytheeattenuation constant for said non-circular electric mode wave energybetween said second and said fourth attenuation constants, said firstand said third attenuation constants being substantially equal.

2. Means for selectively attenuating high frequency non-circularelectric mode wave energy comprising a first elongated member ofconducting material wound in a substantially helical form having equallyspaced turns, a second member of conducting material wound over alongitudinal region of said first member in a substantially helical formhaving progressively increasing spaces between successive turns, and acasing of dissipative material surrounding said helices.

3. The combination according to claim 2 wherein the diameter of saidsecond conducting material is approximately four times the diameter ofsaid first conducting material.

4. The combination according to claim 2 wherein said second helixextends longitudinally along said first helix firom between one-half toone beat wavelength where a beat wavelength A is defined as A being thewavelength in the attenuator of an incident non-circular electric modeand x being the wavelength in the attenuator of a spuriously generatedmode.

5. The combination according to claim 2 wherein the maximum distancebetween successive turns of said second helix is approximately one-halfwavelength of the lowest non-circular mode'to be applied to saidattenuator.

6. A high frequency electromagnetic wave mode filter comprising a firsthelix of insulated closely spaced turns for the propagation therethroughof circular electric wave energy, second and third helical membersconductively insulated from each other and from said first helix havingnon-uniform spacing between turns thereof wound directly over said firsthelix each of said members extending longitudinally from an end to aregion towards the center of said first helix being close wound at saidends and having a progressively increasing pitch, said membersterminating when the space between successive turns has increased toapproximately one-half wavelength of the lowest frequency non-circularelectric mode to be filtered therein, and a high loss outer jacketsurrounding said helices.

7. The combination according to claim 1 wherein said third meanscomprises an elongated member of conducting material wound over alongitudinal region of said second propagating means in a substantiallyhelical form having progressively increasing spaces between successiveturns, and a casing of dissipative material surrounding said helix.

8. In an electromagnetic wave transmission system propagating circularelectric and non-circular electric j mode wave energy, a selective modefilter for the preferential transmission of said circular electric waveenergy comprising an elongated member of' conducting material Mound-in asubstantially helical form having-.equally spaced turns, saidhelixhaving a first and a second at- 7 :tenuation constant for said circularand said non-circular electric mode wave energy, respectively, a secondtransmission path having third and fourth attenuation constants,respectively, for said circular and said non-circular electric mode waveenergy comprising a conductively bounded circular cylindrical pipe, andmeans coupled to said helix and to said pipe to vary gradually theattenuation constant for said non-circular electric mode wave energybetween said second and said fourth attenuation beingsubst'antiallyequal. a I, 9. An electromagnetic wave transmission system adapted forpropagating circular electric and non-circular .electn'c mode waveenergy comprising a section of solid conductive pipe connected at onepoint to a transmission onstants, said first and said third attenuationconstants medium comprising an elongated member of "conducting materialwound in the form of a helix, 'said piperand said helix having lowattenuationconstants for both of said modes, means "forintroducing'aregion of'high atsurrounding said helix adjacent said onepoint for gradually exposing said non-circular, mode to said dissipativematerial thereby gradually increasing the attenuation constant presentedto said non-circular modes from said 3 low attenuation constant to saidhigh attenuation constant. 7

References Cited in the file of this patent V UNITED STATES PATENTS2,343,790 Cutler j July 15, 1958 7 FOREIGN PATENTS France V Mar. 19,1956

